I took a quick poke and this seems to be a d3dx9 issue. If you use the Windows native version of d3dx9_35.dll it should go away.
Yeah, just tested it and can confirm that native d3dx9_35.dll works around it. I can file a Wine bug for it - I hope they can accept it even though it's exposed by dxvk use case.
Closing since this is tracked on the Wine tracker
I noticed that quite for a long time already in some scenes in the Witcher there are glitches when using dxvk.
Here is a small video example (see Shani's hair and some general flickering and light anomalies especially around that candle on the table):
witcher_bug.webm
Save file:
witcher_save.zip
Set up:
Please let me know if some kind of Vulkan captures could help narrow this down, I'll try to add more info if needed.